As has been discussed on this blog, it appears that the danger of radiation does not come from the core of the Fukushima reactors but from cooling ponds used to store used fuel rods. Without being constantly doused with cool water, these rods will overheat or catch fire and begin discharging radiation into the atmosphere.

The water in the pools must be constantly circulated to remain cool, and as the earthquake destroyed the circulation system, the rods are now over heating. The rescue mission’s objective is to cool the rods to a manageable level. The picture directly below is believed to be of the damaged cooling pools. The information below that comes from the site of The International Atomic Energy Agency.

Spent fuel that has been removed from a nuclear reactor generates intense heat and is typically stored in a water-filled pool to cool it and provide protection from its radioactivity. The water in the pool is continuously cooled to remove heat produced by spent fuel assemblies. According to IAEA experts, a typical spent fuel pool temperature is kept below 25 ˚C under normal operating conditions. The temperature of a spent fuel pool is maintained by constant cooling, which requires a constant power source.

Given the intense heat and radiation that spent fuel assemblies can generate, spent fuel pools must be constantly checked for water level and temperature. If fuel is no longer covered by water or temperatures reach a boiling point, fuel can become exposed and create a risk of radioactive release. The concern about the spent fuel pools at Fukushima Daiichi is that sources of power to cool the pools may have been compromised.

The IAEA can confirm the following information regarding the temperatures of the spent nuclear fuel pools at Units 4, 5 and 6 at Fukushima Daiichi nuclear power plant:

Unit 4
14 March, 10:08 UTC: 84 ˚C
15 March, 10:00 UTC: 84 ˚C
16 March, 05:00 UTC: no data
Unit 5
14 March, 10:08 UTC: 59.7 ˚C
15 March, 10:00 UTC: 60.4 ˚C
16 March, 05:00 UTC: 62.7 ˚C
Unit 6
14 March, 10:08 UTC: 58.0 ˚C
15 March, 10:00 UTC: 58.5 ˚C
16 March, 05:00 UTC: 60.0 ˚C

The IAEA is continuing to seek further information about the water levels, temperature and condition of all spent fuel pool facilities at the Fukushima Daiichi nuclear power plant. (thanks to commenter Bez for the pic)